Laser production and deposition of light-emitting silicon nanoparticles

Silicon clusters and nanoparticles are produced by CO2-laser-induced decomp osition of silane in a flow reactor. In contrast to conventional techniques , the particles are expanded, directly after production, through a conical nozzle into a high vacuum chamber and then transferred into a molecular bea m apparatus where they an analyzed in situ by time-of-flight mass spectrome try (TOF-MS). The analysis reveals that the flow reactor emits, besides sma ll clusters, also high-purity silicon crystallites with diameters between 2 and 20 nm. It is found that the particles' velocity strongly correlates wi th their mass. This feature and the fact that the particles are produced in the pulsed mode enable us, by introducing a chopper into the cluster beam, to considerably reduce the dispersion of their size distribution and to pe rform size-selected low-energy cluster deposition on various substrates. Hi gh-resolution electron transmission micrographs demonstrate the capabilitie s of the new apparatus and reveal interesting details of the crystalline st ructure of silicon nanoparticles as a function of their size. The monodispe rsed silicon films have been further characterized by studying their lumine scence and Raman scattering behavior. As predicted by theoretical models, t he peak of the luminescence curve shifts with decreasing particle size to s maller wavelengths (higher energies). Structured thin films are obtained by shaping the duster beam with a mask and depositing the nanocrystals at low energy on a sapphire substrate. Upon illumination with ultraviolet radiati on, the structured film exhibits strong photoluminescence (PL) in the red. (C) 2000 Elsevier Science B.V. All rights reserved.